Image processing apparatus, image processing method and storage medium

ABSTRACT

An image processing apparatus that generates image data to be output to an image forming apparatus printing an image, and includes: a generation unit configured to generate, based on an input image, first generation amount data indicating a generation amount of each of one or more kinds of ink dot for each pixel and second generation amount data indicating a generation amount of a blank dot for which the ink dot is not formed for each pixel; and a processing unit configured to determine a dot arrangement pattern indicating an arrangement of each of the one or more kinds of ink dot by performing quantization processing using the first generation amount data and the second generation amount data.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of International Patent ApplicationNo. PCT/JP2019/002485, filed Jan. 25, 2019, which claims the benefit ofJapanese Patent Application Nos. 2018-011493 filed Jan. 26, 2018 and2019-004549 filed Jan. 15, 2019, all of which are hereby incorporated byreference herein in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to quantization processing to convert thenumber of tones of an input image into a smaller number of tones forforming an image by using a color material represented by ink and thelike.

Background Art

Conventionally, as a technique to print an image processed by a computeror the like in a multi-color and multi-tone manner, an image formingapparatus that forms an image on a printing medium, such as paper, byforming dots by a color material, such as ink, is used widely. For theimage forming apparatus such as this, aiming at improvement of theprinting speed and high image quality, a variety of methods relating todot arrangement control have been proposed. For example, PatentLiterature 1 has disclosed a technique to cause dots of deep and lightinks to be highly dispersive by determining the dot arrangement in orderfrom the ink whose density is the highest in an ink jet printingapparatus using inks whose density is different for the same color.Further, Patent Literature 2 has disclosed a technique to determine thedot arrangement so that the arrangement of each dot whose formable sizeis different becomes highly dispersive in total.

CITATION LIST Patent Literature

Patent Literature 1: International Publication No. WO98/03341

Patent Literature 2: Japanese Patent Laid-Open No. 2007-6391

In Patent Literatures 1 and 2 described above, the arrangement of dotsfor which ink is ejected is determined by performing quantizationprocessing by the error diffusion method for the multi-valued ink valuedata corresponding to a plurality of kinds of dots. In theseconventional techniques, the pixel corresponding to the paper whiteportion to which no ink is ejected is determined as a portion in whichany dot is not arranged. As a result of that, for example, in PatentLiterature 1, although a high dispersity is obtained for dots of thedeep ink and the light ink, respectively, there is a case where thedispersity is not good for the paper white portion, which is not thecontrol target. Further, in Patent Literature 2, although a highdispersity is obtained in the dot arrangement in total of each of thelarge, medium, and small dots, there is a case where a sufficientdispersity is not obtained on the whole including the paper whiteportion. In a case where attention is focused on the pixel correspondingto the paper white portion as described above, it happens sometimes thata good granularity is not implemented by the conventional technique andthe image quality deteriorates.

The present invention has been made in view of the above-describedproblem and an object is to enable quantization processing capable ofobtaining a better granularity.

SUMMARY OF THE INVENTION

The image processing apparatus according to the present invention is animage processing apparatus that generates image data to be output to animage forming apparatus printing an image by an ink dot formed byejecting ink onto a printing medium, and includes: a generation unitconfigured to generate, based on an input image, first generation amountdata indicating a generation amount of each of one or more kinds of inkdot for each pixel and second generation amount data indicating ageneration amount of a blank dot for which the ink dot is not formed foreach pixel; and a processing unit configured to determine a dotarrangement pattern indicating an arrangement of each of the one or morekinds of ink dot by performing quantization processing using the firstgeneration amount data the second generation amount data.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram showing an example of a configuration of an imageforming apparatus and FIG. 1B is a block diagram showing a main hardwareconfiguration of an image processing unit.

FIG. 2 is a functional block diagram showing an internal configurationof a separation processing unit and a quantization processing unitaccording to a first embodiment.

FIG. 3 is a flowchart showing a flow of processing within the imageprocessing unit according to the first embodiment.

FIG. 4 is a diagram showing an example of a dot separation tableaccording to the first embodiment.

FIG. 5A to FIG. 5I are diagrams explaining effects in a case wherearrangement of blank dots is determined with priority in a dark tone.

FIG. 6 is a flowchart showing details of quantization processingaccording to the first embodiment.

FIG. 7 is an explanatory diagram of error diffusion coefficients.

FIG. 8A to FIG. 8D are each a diagram showing an example of an errorline buffer.

FIG. 9 is a flowchart showing details of quantization processingaccording to a second embodiment.

FIG. 10A to FIG. 10C are each a diagram showing an example of an errorline buffer.

FIG. 11A and FIG. 11B are diagrams explaining effects of quantizationprocessing by a method of the second embodiment.

FIG. 12 is a functional block diagram showing an internal configurationof a separation processing unit and a quantization processing unitaccording to a third embodiment.

FIG. 13 is a flowchart showing a flow of processing within an imageprocessing unit according to the third embodiment.

FIG. 14 is a diagram showing an example of a dot separation tableaccording to the third embodiment.

FIG. 15 is a flowchart showing details of quantization processingaccording to a fourth embodiment.

FIG. 16 is a flowchart showing details of quantization processingaccording to a fifth embodiment.

FIG. 17 is a functional block diagram showing an internal configurationof a separation processing unit and a quantization processing unitaccording to a sixth embodiment.

FIG. 18 is a flowchart showing a flow of processing within an imageprocessing unit according to the sixth embodiment.

FIG. 19 is a flowchart showing details of quantization processingaccording to the sixth embodiment.

FIG. 20A to FIG. 20C are explanatory diagrams of quantization processingby a dither method.

FIG. 21 is a functional block diagram showing an internal configurationof a separation processing unit and a quantization processing unitaccording to a seventh embodiment.

FIG. 22 is a flowchart showing a flow of processing within an imageprocessing unit according to the seventh embodiment.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, with reference to the attached drawings, the presentinvention is explained in detail in accordance with preferredembodiments. Configurations shown in the following embodiments aremerely exemplary and the present invention is not limited to theconfigurations shown schematically.

First Embodiment (Configuration of Printing System)

FIG. 1A is a diagram showing an example of the configuration of an imageforming apparatus that performs printing by the ink jet method accordingto the present embodiment. An image forming apparatus 100 of the presentembodiment includes an image processing unit 110 and an image formingunit 120. Upon receipt of printing instructions of a user, the imageprocessing unit 110 generates dot arrangement data corresponding to nkinds of dot (in the present embodiment, three kinds of dot, that is,large, medium, and small dots) for each ink color and the image formingunit 120 performs printing based on the dot arrangement data.

FIG. 1B is a block diagram showing the main hardware configuration ofthe image processing unit 110. The image processing unit 110 includes aCPU 111, a RAM 112, a ROM 113, a separation processing unit 114, and aquantization processing unit 115. The CPU 111 centralizedly controls theentire image processing unit 110 based on programs stored in the ROM113. The RAM 112 is used as a work area of the CPU 111. The separationprocessing unit 114 generates (n (n is an integer not less than 1 and inthe present embodiment, n=3)+1) pieces of dot generation amount data oneach ink color from printing-target input image data. The dot generationamount data is data in which a value indicating tone information issaved for each pixel. The quantization processing unit 115 performsquantization processing (halftone processing) by using the (n+1) piecesof dot generation amount data input from the separation processing unit114 and generates n pieces of dot arrangement data on each ink color,which are represented by a smaller number of tones. Each piece of dotarrangement data is data in which information indicating whether or nota dot is arranged is saved for each pixel. Details of the separationprocessing unit 114 and the quantization processing unit 115 will bedescribed later. The above-described two processing units (separationprocessing unit 114 and quantization processing unit 115) configuringthe image processing unit 110 are enabled to perform high-speedprocessing by configuring them by logic circuits. The image processingunit 110 may include components other those described above, such aprocessing unit configured to perform y correction and a processing unitconfigured to perform color matching, but they are not the main purposeof the present invention, and therefore, explanation is omitted.Further, it may also be possible to configure the image processing unit110 as an apparatus independent of the image forming apparatus, forexample, such as a general personal computer. In this case, by a printerdriver installed in the apparatus, the functions of the above-describedimage processing unit 110 may be implemented.

The image forming unit 120 prints an image on a printing medium, such aspaper, by using color materials in, for example, four colors of CMYK(cyan, magenta, yellow, black). Although the printing method of an imageis not limited in particular, in the present embodiment, explanation isgiven by taking the ink jet method as an example. In a case of the inkjet method, an image is formed by ejecting ink droplets in each colorfrom a line head on which nozzle columns provided for each color of CMYKare arranged side by side. The line head is, for example, a long head(also called a page-wide head or a full-line type head) comprising anozzle array covering the entire range of a drawing area in the widthdirection perpendicular to the sheet conveyance direction. In a case ofthe long head, the long head is arranged so as to extend in thedirection substantially perpendicular to the sheet conveyance directionand by relatively moving a sheet once (single-pass method) or aplurality of times (multi-pass method) with respect to the head, animage with a predetermined resolution is formed. Within the nozzle ofthe line head, a piezo element is provided and the amount of ink to beejected from an opening of the nozzle is adjusted by the voltage to beapplied to the piezo element. A variety of types of ink head exist andit is needless to say that the long head is an example and the head isnot limited to the long head.

In the present embodiment, the image forming unit 120 is configured toas to be capable of forming dots of three different sizes in each colorof CMYK. In the present specification, the dot that is formed by inkbeing ejected actually from the nozzle is collectively referred to as“actual dot (or ink dot)” and the dot is called “large dot”, “mediumdot”, and “small dot” in order from the dot whose amount of ejected inkis the largest. By four kinds of dot, that is, the actual large, medium,and small dots plus a dot for which no ink is ejected from the nozzle,that is, whose amount of ejection is zero, it is made possible torepresent four density levels in one nozzle. In the presentspecification, the dot corresponding to the paper white portion, whichis formed by ejecting no ink, is called “blank dot (or zero dot)”. Thehead that ejects ink is not limited to the piezo ejection method and itmay also be possible to adopt the thermal ejection method. With thethermal ejection method, it is possible to design a configuration inwhich ink at three different density levels can be ejected by comprisinga plurality of line heads whose amount of ejection is different from oneanother.

(Separation Processing Unit and Quantization Processing Unit)

Following the above, the separation processing unit 114 and thequantization processing unit 115, which are features of the presentembodiment, are explained in detail. FIG. 2 is a functional blockdiagram showing the internal configuration of the separation processingunit 114 and the quantization processing unit 115. As shown in FIG. 2,the separation processing unit 114 has a color separation processingunit 201, an actual dot generation amount determination unit 202, and ablank dot generation amount determination unit 203. Further, thequantization processing unit 115 has a priority order determination unit204, a dot arrangement determination unit 205, and a data output unit206. In the following, explanation is given along the flowchart shown inFIG. 3. In the following explanation, symbol “S” represents a step.

At S301, printing-target image data received by the image formingapparatus 100 from an external information processing apparatus (notshown schematically) or the like is input to the color separationprocessing unit 201 of the separation processing unit 114. This inputimage data is, for example, multi-valued RGB image data in which threecolor components of red (R), green (G), and blue (B) are represented in,for example, 256 tones. At S302 that follows, the color separationprocessing unit 201 performs color separation processing for the RGBimage data by using a color separation table prepared in advance andconverts the RGB image data into image data for each of CMYKcorresponding to each ink color (image plane corresponding to each inkcolor of CMYK). The obtained image data for each of CMYK (hereinafter,also described collectively as CMYK image data) is sent to the actualdot generation amount determination unit 202.

At step 303, the actual dot generation amount determination unit 202acquires dot generation amount data for each size of the large, medium,and small dots by performing dot size separation processing for eachpiece of CMYK image data by using a dot separation table prepared inadvance. Specifically, the actual dot generation amount determinationunit 202 separates the image plane corresponding to each color of CMYKinto data (each dot plane of large, medium, and small dots) for eachactual dot of the large, medium, and small dots whose ink ejectionamount is different, which corresponds to one dot. FIG. 4 is a diagramshowing an example of the dot separation table according to the presentembodiment. The horizontal axis represents the tone value in the imageplane corresponding to each ink color after the color separationprocessing and the vertical axis represents the generation amount (ratioof dots formed per unit area) for each actual dot of the large, medium,and small dots. In a case of FIG. 4, the maximum value of both thevertical axis and the horizontal axis is 255. The dot separation tablesuch as this is prepared and stored in advance, for example, for eachcolor of CMYK, and read and referred to at this step. For example, it isassumed that FIG. 4 is the dot separation table of cyan and the tonevalue in the cyan image plane (C plane) is “200”. In this case, in the Cplane, the generation amount of the large dot is “145”, the generationamount of the medium dot is “75”, and the generation amount of the smalldot is “18”. In this manner, for the C plane, data C_large indicatingthe generation amount of the large dot, data C_medium indicating thegeneration amount of the medium dot, and data C_small indicating thegeneration amount of the small dot are generated. For magenta, yellow,and black, similar dot generation amount data is generated. That is, forthe M plane, M_large, M_medium, and M_small, for the Y plane, Y_large,Y_medium, and Y_small, and for the K plane, K_large, K_medium, andK_small are generated. The data (a total of 12 dot planes) indicatingthe generation amounts of the actual large, medium, and small dots foreach of CMYK thus generated is sent to the quantization processing unit115.

At S304, the blank dot generation amount determination unit 203generates dot generation amount data on the blank dot for each imageplane of CMYK. In the present embodiment, an input image is representedby combining the three kinds of actual dot, that is, the large, medium,and small dots, in accordance with the tone value in each color of CMYK.Consequently, the theoretical maximum value of the total of the dotgeneration amounts of the three kinds of actual dot in each pixel is255. Because of this, it is possible to regard the difference betweenthe maximum value 255 and the total of the dot generation amounts of thethree kinds of actual dot as the tone value (corresponding to thegeneration amount) to be represented by the blank dot. Consequently,data C_zero, M_zero, Y_zero, and K_zero each indicating the blank dotgeneration amount in each image plane of CMYK is expressed by formula(1) to formula (4) below, respectively.

C_zero=255−(C_large+C_medium+C_small)  formula (1)

M_zero=255−(M_large+M_medium+M_small)  formula (2)

Y_zero=255−(Y_large+Y_medium+Y_small)  formula (3)

K_zero=255−(K_large+K_medium+K_small)  formula (4)

The data (zero dot plane) indicating the blank dot generation amount ineach color obtained as described above is sent to the quantizationprocessing unit 115. Then, by using the dot generation amount data onthe four kinds (large, medium, and small dots plus zero dot) for each ofCMYK, at next S305, the dot arrangement of each color of CMYK isdetermined. In the present embodiment, in the dark tone (shadow area),control is performed so that the arrangement of the blank dot isdetermined prior to the arrangement of the actual large, medium, andsmall dots. A low dispersity of the dot arrangement of the paper whiteportion visually conspicuous in the dark tone will be a factor thatdeteriorates the granularity. Because of this, by determining the blankdot arrangement with priority in the dark tone over the actual dotarrangement, a high dispersity is secured. The method of the presentembodiment aims at improvement of the granularity by also generating thedot generation amount data on the blank dot corresponding to the paperwhite portion and determining the blank dot arrangement with priorityover the actual dot arrangement.

At S305, the quantization processing unit 115 generates dot arrangementdata by performing the quantization processing based on the four kinds(large, medium, and small dots plus zero dot) of dot generation amountdata for each of CMYK. At this time, the priority order is determinedfor the above-described four kinds of dot and the dot arrangement isdetermined sequentially in order from the dot whose priority order isthe highest. FIG. 5A to FIG. 5I are diagrams explaining effects in acase where the blank dot arrangement is determined with priority in thedark tone. FIG. 5A to FIG. 5D each show an arrangement example of eachdot in a case where the blank dot arrangement is determined withpriority. Here, the tone value “200” is taken as an example. FIG. 5A isthe blank dot arrangement, FIG. 5B is the small dot arrangement, FIG. 5Cis the medium dot arrangement, and FIG. 5D is the large dot arrangement.In FIG. 5A, the blank dot is a pixel for which no ink is ejectedoriginally, but here, to explicitly indicate the dot arrangement, theposition at which the blank dot is arranged is indicated by the blackdot. In FIG. 5B to FIG. 5D also, as in FIG. 5A, the position at whicheach dot is arranged is indicated by the black dot of the same size.From FIG. 5A, it is known that the dispersity is high because the blankdot arrangement is determined with priority over the arrangement of theother actual dots. On the other hand, from FIG. 5B to FIG. 5D, thearrangement of the actual large, medium, and small dots is determined soas to be exclusive of the blank dot, and therefore, the dispersity isdeteriorated compared to a case where the arrangement of each dot isdetermined with priority. FIG. 5E to FIG. 5H are each an image diagramin a case where ink dots are formed on a sheet with each dot alone basedon the dot arrangement in FIG. 5A to FIG. 5D. As regards the blank dot,the ink ejection amount is zero, and therefore, no dot is formed (FIG.5E). As regards the small dot, the medium dot, and the large dot, ink isejected in each corresponding amount of ejection and the actual dots areformed (FIG. 5F, FIG. 5G FIG. 5H). FIG. 5I is an image diagram in a casewhere the actual large, medium, and small dots are formed on the samesheet based on the dot arrangement in FIG. 5A to FIG. 5D. Thearrangement of the blank dot is determined with priority, and therefore,it is known that the dispersity of the dot arrangement in the visuallyconspicuous paper white portion is high and the image whose granularityis good is obtained. Details of the quantization processing will beexplained in detail by using another flowchart.

(Details of Quantization Processing)

Following the above, details of the quantization processing at S305 areexplained along the flowchart in FIG. 6. This processing is the same foreach color of CMYK, and therefore, in the following, explanation isgiven by taking black (K) as a representative example. Further, in thefollowing, explanation is given by taking a case as an example where thedot generation amount data is converted into binary dot arrangement databy quantizing the dot generation amount data by the error diffusionmethod, but the method of quantization is not limited to the errordiffusion method and for example, the dither method may be used. In thefollowing explanation, symbol “S” represents a step.

At S601, the priority order determination unit 204 sets an unprocessedpixel in the K plane as the pixel of interest and for the pixel ofinterest, determines the priority order for the above-described fourkinds (large, medium, and small dots plus zero dot) of dot generationamount data generated in the separation processing unit 114. Here, it isassumed that the priority order is represented by P1, P2, P3, and P4 inorder from the highest priority order. In this case, in the light tone,the higher the density level of the deep dot (that is, the larger theink dot), the more conspicuous and in the dark tone, the lower thedensity level of the light dot (that is, the smaller the dot (zerodot)), the more conspicuous. Consequently, it is desirable to give ahigher priority order to the dot that is more conspicuous in the tone.Here, as shown in FIG. 4, in general, in the dark tone, the large dotgeneration amount is increased and the paper white portion is reduced.Therefore, by comparing the generation amount of the large dot and thegeneration amount of the blank dot, which are determined in accordancewith the tone value, it is possible to determine whether the tone is thelight tone or the dark tone. Further, the smaller the number of dots,the higher the degree of freedom of the dot arrangement is, and it ispossible to determine the arrangement so as to be highly dispersive.Consequently, by comparing the dot generation amount data on each dotand determining the priority order so that the dots whose number issmaller is given a higher priority, it is possible to effectivelyimprove the dispersity in the arrangement of the conspicuous dot. Basedon the way of thinking as described above, for example, the priorityorder is determined by the method as in 1), 2), or 3) below.

1) Determination Method Based on Dot Generation Amount Data on Large Dotand Zero Dot

First, large dot generation amount data K_large (x, y) and zero dotgeneration amount data K_zero (x, y), which correspond to a pixel ofinterest position (x, y), are read and both are compared. In a casewhere comparison results indicate that K_large (x, y)<K_zero (x, y), itis determined that the pixel of interest belongs to the light tone andthe priority order in the pixel of interest is determined as P1=K_large,P2=K_medium, P3=K_small, and P4=K_zero. On the other hand, in a casewhere K_large (x, y)≥K_zero (x, y), it is determined that the pixel ofinterest belongs to the dark tone and the priority order in the pixel ofinterest is determined as P1=K_zero, P2=K_small, P3=K_medium, andP4=K_large.

2) Determination Method Based on Only Dot Generation Amount Data onLarge Dot

First, the large dot generation amount data corresponding to the pixelof interest position (x, y) is referred to. Then, in a case whereK_large (x, y)=0, it is determined that the pixel of interest belongs tothe light tone and the priority order in the pixel of interest isdetermined as P1=K_large, P2=K_medium, P3=K_small, and P4=K_zero. On theother hand, in a case where K_large (x, y)>0, it is determined that thepixel of interest belongs to the dark tone and the priority order in thepixel of interest is determined as P1=K_zero, P2=K_small, P3=K_medium,and P4=K_large.

3) Determination Method Based on Dot Generation Amount Data on Large,Medium, Small, and Zero Dots

The large, medium, and small dots generation amount data correspondingto the pixel of interest position (x, y) is referred to. Then, thepriority order in the pixel of interest is determined as P1, P2, P3, andP4 in order from the smallest dot generation amount data. For example,in a case where K_large (x, y)<K_zero (x, y)<K_small (x, y)<K_medium (x,y), the priority order is determined as P1=K_large, P2=K_zero,P3=K_small, and P4=K_medium.

By either of the determination methods in 1) and 2) described above, itis possible to give priority to the blank dot arrangement for the pixelof interest belonging to the dark tone. Further, by the determinationmethod in 3) also, it is possible to effectively improve the dispersityin the dot arrangement of the conspicuous dot by determining thepriority order so that the dots whose number of dots is smaller is givena higher priority.

Here, explanation is continued on the assumption that the pixel ofinterest belongs to the dark tone and P1=K_zero, P2=K_small,P3=K_medium, and P4=K_large are determined. The information on thepriority order (hereinafter, priority order information) thus determinedis referred to by the dot arrangement determination unit 205 and thedata output unit 206. Explanation of FIG. 6 is returned.

At S602 and S603 that follow, the dot arrangement determination unit 205determines the dot arrangement by performing the quantization processingby the error diffusion method for the dot generation amount data (here,K_zero) on P1 whose priority order is the highest based on the priorityorder information. FIG. 7 shows an example of coefficients to be used atthe time of diffusing errors to the peripheral pixels by the errordiffusion method. In the present embodiment, it is assumed that thereare four coefficients K1 to K4 and for example, K1= 7/16, K2= 3/16, K3=5/16, and K4= 1/16. It is needless to say that the above-described errordiffusion coefficients are an example and the example is not limited tothis. In order to diffuse and accumulated errors by the error diffusioncoefficients such as those, the dot arrangement determination unit 205comprises (n+1) error line buffers corresponding to the (n+1) kinds ofdot, that is, n kinds of actual dots (here, n=3 because of large,medium, and small dots) plus the blank dot. By having the error linebuffer corresponding to each kind of dot, it is possible to keep thecontinuity of the dot pattern for each kind of dot even in a case wherethe priority order changes. FIG. 8A to FIG. 8D each show an example ofthe error line buffer of the present embodiment. FIG. 8A is the errorline buffer corresponding to the large dot, FIG. 8B is the error linebuffer corresponding to the medium dot, FIG. 8C is the error line buffercorresponding to the small dot, and FIG. 8D is the error line buffercorresponding to the blank dot. Each error line buffer has (W (number ofhorizontal pixels in the input image)+1) storage areas (E_K_large (x),E_K_medium (x), E_K_small (x), and E_K_zero (x), here, x=1 to (W+1)).Then, in each storage area, the quantization error is stored by amethod, to be described later. All the error line buffers areinitialized by an initial value (=0) or a random value before the startof the processing. Here, it is assumed that each error line buffer isrepresented by E_P1, E_P2, E_P3, and E_P4 in order from the highestpriority order. Here, the priority order is P1=K_zero, P2=K_small,P3=K_medium, and P4=K_large. Consequently, E_P1=E_K_zero,E_P2=E_K_small, E_P3=E_K_medium, and E_P4=E_K_large will result.

At S602, for dot generation amount data P1 (x, y) corresponding to thepixel of interest position (x, y), its corresponding error E_P1 (x) isread from the error line buffer and added thereto. Dot generation amountdata P1′ after the error is added is expressed by formula (5) below.

P1′=P1(x,y)+E_P1(x)  formula(5)

Then, at S603, quantization by a comparison between P1′ and a thresholdvalue Th is performed. In a case where P1′ is greater than the thresholdvalue Th, the quantization results are ON (=255) and in a case where P1′is less than or equal to the threshold value Th, the quantizationresults are OFF (=0). Here, it is assumed that the quantization resultsfor P1′ are represented as O_P1. In this manner, in the dark tone, theblank dot arrangement corresponding to P1 is determined with priorityover the actual dot arrangement.

Next, at S604 to S606, the dot arrangement determination unit 205performs quantization processing for the dot generation amount data(here, K_small) on P2 whose priority order is the second highest anddetermines the dot arrangement.

First, at S604, a correction amount H based on the arrangementdetermination results of the dot whose priority order is higher (here,the blank dot) is calculated. Then, processing to reflect the obtainedcorrection amount H in dot generation amount data P2 (x, y)corresponding to the pixel of interest position (x, y) is performed. Inthis case, the correction amount H is expressed by formula (6) below anddot generation amount data P2′ after the correction amount H isreflected is expressed by formula (7) below.

H=P1(x,y)−O_P1(x,y)  formula(6)

P2′=P2(x,y)+H  formula(7)

By the above-described processing, it is made possible to exclusivelyobtain the quantization results of the dot (here, small dot)corresponding to P2 for the arrangement of the dot corresponding to P1whose priority order is higher.

Next, at S605, for the dot generation amount data P2 (x, y)corresponding to the pixel of interest position (x, y), itscorresponding error E_P2 (x) is read from the error line buffer andadded thereto. Dot generation amount data P2″ after the error is addedis expressed by formula (8) below.

P2″=P2′+E_P2(x)  formula(8)

Then, at S606, quantization by a comparison between P2″ and thethreshold value Th is performed. In a case where P2″ is greater than thethreshold value Th, the quantization results are ON (=255) and in a casewhere P2″ is less than or equal to the threshold value Th, thequantization results are OFF (=0). Here, it is assumed that thequantization results for P2″ are represented as O_P2. Next, at S607 toS609, the dot arrangement determination unit 205 performs quantizationprocessing for the dot generation amount data (here, K_medium) on P3whose priority order is the third highest and determines the dotarrangement.

First, at S607, the correction amount H based on the arrangementdetermination results of the dots whose priority order is higher (here,the blank dot and the small dot) is updated. Then, processing to reflectthe obtained correction amount H in dot generation amount data P3 (x, y)corresponding to the pixel of interest position (x, y) is performed. Inthis case, the correction amount H after the updating is expressed byformula (9) below and dot generation amount data P3′ after thecorrection amount H is reflected is expressed by formula (10) below.

H=H+P2(x,y)−O_P2(x,y)  formula(9)

P3′=P3(x,y)+H  formula(10)

By the above-described processing, it is made possible to exclusivelyobtain the quantization results of the dot (here, medium dot)corresponding to P3 for the arrangement of the dots corresponding to P1and P2 whose priority order is higher.

At S608, for the dot generation amount data P3 (x, y) corresponding tothe pixel of interest position (x, y), its corresponding error E_P3 (x)is read from the error line buffer and added thereto. Dot generationamount data P3″ after the error is added is expressed by formula (11)below.

P3″=P3′+E_P3(x)  formula(11)

Then, at S609, quantization by a comparison between P3″ and thethreshold value Th is performed. In a case where P3″ is greater than thethreshold value Th, the quantization results are ON (=255) and in a casewhere P3″ is less than or equal to the threshold value Th, thequantization results are OFF (=0). Here, it is assumed that thequantization results for P3″ are represented as O_P3. Next, at S610 toS612, the dot arrangement determination unit 205 performs quantizationprocessing for the dot generation amount data (here, K_large) on P4whose priority order is the fourth highest and determines the dotarrangement.

First, at S610, the correction amount H based on the arrangementdetermination results of the dots whose priority order is higher (here,the blank dot, the small dot, and the medium dot) is updated. Then,processing to reflect the obtained correction amount H in dot generationamount data P4 (x, y) corresponding to the pixel of interest position(x, y) is performed. In this case, the correction amount H after theupdating is expressed by formula (12) below and dot generation amountdata P4′ after the correction amount H is reflected is expressed byformula (13) below.

H=H+P3(x,y)−O_P3(x,y)  formula(12)

P4′=P4(x,y)+H  formula (13)

By the above-described processing, it is made possible to exclusivelyobtain the quantization results of the dot (here, large dot)corresponding to P4 for the arrangement of the dots corresponding to P1to P3 whose priority order is higher.

At S611, for the dot generation amount data P4 (x, y) corresponding tothe pixel of interest position (x, y), its corresponding error E_P4 (x)is read from the error line buffer and added thereto. Dot generationamount data P4″ after the error is added is expressed by formula (14)below.

P4″=P4′+E_P4(x)  formula (14)

Then, at S612, quantization by a comparison between P4″ and thethreshold value Th is performed. In a case where P4″ is greater than thethreshold value Th, the quantization results are ON (=255) and in a casewhere P4″ is less than or equal to the threshold value Th, thequantization results are OFF (=0). Here, it is assumed that thequantization results for P4″ are represented as O_P4. After this, atS613, differences between the values P1″, P2″, P3″, and P4″ before thequantization processing is performed and each of the quantizationresults O_P1, O_P2, O_P3, and O_P4 are calculated. In a case where thedifferences calculated here are taken to be ERR_P1, ERR_P2, ERR_P3, andERR_P4, they are expressed by formula (15) to formula (18) below.

ERR_P1=P1″−O_P1  formula (15)

ERR_P2=P2″−O_P2  formula (16)

ERR_P3=P3″−O_P3  formula (17)

ERR_P4=P4″−O_P  formula (18)

Then, at S614, the calculated errors are diffused to the pixels aroundthe pixel of interest in accordance with the above-described errordiffusion coefficients. Specifically, the errors ERR_P1, ERR_P2, ERR_P3,and ERR_P4 in P1 to P4 are stored in each corresponding line buffer asfollows.

First, the error ERR_P1 corresponding to P1 is stored in an error linebuffer E_K_zero of K_zero as follows because E_P1=E_K_zero.

E_K_zero(x+1)=E_K_zero(x+1)+ERR_P1× 7/16(x<W)

E_K_zero(x−1)=E_K_zero(x−1)+ERR_P1× 3/16(x>1)

E_K_zero(x)=E_K_zero(W+1)+ERR_P1× 5/16(1<x<W)

E_K_zero(x)=E_K_zero(W+1)+ERR_P1× 8/16(x=1)

E_K_zero(x)=E_K_zero(W+1)+ERR_P1× 13/16(x=W)

E_K_zero(W+1)=ERR_P1× 1/16(x<W)

E_K_zero(W+1)=0(x=W)

The error ERR_P2 corresponding to P2 is stored in an error line bufferE_K_small of K_small as follows because E_P2=E_K_small.

E_K_small(x+1)=E_K_small(x+1)+ERR_P2× 7/16(x<W)

E_K_small(x−1)=E_K_small(x−1)+ERR_P2× 3/16(x>1)

E_K_small(x)=E_K_small(W+1)+ERR_P2× 5/16(1<x<W)

E_K_small(x)=E_K_small(W+1)+ERR_P2× 8/16(x=1)

E_K_small(x)=E_K_small(W+1)+ERR_P2× 13/16(x=W)

E_K_small(W+1)=ERR_P2× 1/16(x<W)

E_K_small(W+1)=0(x=W)

The error ERR_P3 corresponding to P3 is stored in an error line bufferE_K_medium of K_medium as follows because E_P3=E_K_medium.

E_K_medium(x+1)=E_K_medium(x+1)+ERR_P3× 7/16(x<W)

E_K_medium(x−1)=E_K_medium(x−1)+ERR_P3× 3/16(x>1)

E_K_medium(x)=E_K_medium(W+1)+ERR_P3× 5/16(1<x<W)

E_K_medium(x)=E_K_medium(W+1)+ERR_P3× 8/16(x=1)

E_K_medium(x)=E_K_medium(W+1)+ERR_P3× 13/16(x=W)

E_K_medium(W+1)=ERR_P3× 1/16(x<W)

E_K_medium(W+1)=0(x=W)

The error ERR_P4 corresponding to P4 is stored in an error line bufferE_K_large of K_large as follows because E_P4=E_K_large.

E_K_large(x+1)=E_K_large(x+1)+ERR_P4× 7/16(x<W)

E_K_large(x−1)=E_K_large(x−1)+ERR_P4× 3/16(x>1)

E_K_large(x)=E_K_large(W+1)+ERR_P4× 5/16(1<x<W)

E_K_large(x)=E_K_large(W+1)+ERR_P4× 8/16(x=1)

E_K_large(x)=E_K_large(W+1)+ERR_P4× 13/16(x=W)

E_K_large(W+1)=ERR_P4× 1/16(x<W)

E_K_large(W+1)=0(x=W)

At S615, the data output unit 206 outputs dot arrangement datacorresponding to each of the large, medium, and small dots that the linehead can form based on the priority order information received from thepriority order determination unit 204. Here, in the priority orderinformation, P1=K_zero, P2=K_small, P3=K_medium, and P4=K_large. In thiscase, as the dot arrangement data for K ink ejection to the pixel ofinterest position (x, y), the following three pieces of data are outputto the image forming unit 120.

-   -   Large dot: O_P4 (x, y)    -   Medium dot: O_P3 (x, y)    -   Small dot: O_P2 (x, y)

Then, at S616, whether or not the processing for all the pixels of the Kplane is completed is determined. In a case where there is anunprocessed pixel, the processing returns to S601, and the next pixel istaken as the pixel of interest and the processing is continued. On theother hand, in a case where the processing of all the pixels iscompleted, this processing is terminated. The above is the contents ofthe quantization processing by the method of the present embodiment.

Modification Example

In the present embodiment, explanation is given by premising theconfiguration in which the four density levels can be represented by onenozzle for each color of CMYK by the four kinds of dot, that is, theactual dots classified into three sizes of the large, medium, and smalldots plus the blank dot. However, it is also possible to apply thepresent embodiment to an image forming apparatus that performs tonerepresentation by using a plurality of inks having the same hue butdifferent in density (for example, normal ink and light ink) andcombining two kinds of ink dot (deep dot and light dot) and the blankdot. In this case, the above-described processing is performed for thethree kinds of dot, that is, the actual dots classified into the deepdot and the light dot plus the blank dot, and based on dot generationamount data on the three kinds of dot, that is, the deep dot, the lightdot, and the zero dot, and priority order information, the dotarrangement is performed. At this time, the configuration is designed sothat the dot generation amount data on the deep and light dots isgenerated by using a separation table in which the tone value and thedot generation amounts of the two kinds of actual dot, that is, the deepdot and the light dot, are associated with each other in the “actual dotgeneration amount determination unit”. Then, the data indicating thegeneration amount of the blank dot for each image plane in accordancewith the ink color is found based on formula (1) to formula (4)described previously. In this case, for example, for cyan, by taking thedot generation amount of the deep dot to be C_deep and the dotgeneration amount of the light dot to be C_light, the dot generationamount C_zero of the blank dot is expressed by formula (1)′ below.

C_zero=255−(C_deep+C_light)  formula (1)′

In the present embodiment, the paper white portion for which no ink isejected is regarded as the blank dot and the generation amount data onthe blank dot per unit area is generated based on the generation amountdata on the actual dots and quantization is performed. By performingquantization by the method such as this, it is possible to take intoconsideration the arrangement of the blank dot not formed by the colormaterial at the time of obtaining the dot arrangement data on the actualdots different in the density level. Further, for the pixel belonging tothe dark tone within the input image, the dot arrangement of the blankdot is determined with the highest priority, and therefore, it ispossible make the paper white portion conspicuous in the shadow areahighly dispersive.

The dot arrangement according to the present embodiment described aboveis explained. Considering a case where the present embodiment is appliedto an image in which all the pixels have the same pixel value, thedispersity of the dot arrangement of the large dot is the highest on acondition that the image plane is an image of the light tone. On theother hand, in a case where the image place is an image of the darktone, the dispersity of the arrangement of the paper white portion(blank dot) is higher than that of the dot arrangement of the actualdots and the highest. Due to this, it is possible to suppress thegranularity of the paper white portion from becoming conspicuous in thedark tone and the image quality from deteriorating.

Second Embodiment

In the first embodiment, the priority order at the time of the dotarrangement of the four kinds of dot (large, medium, and small dots pluszero dot) is determined for each pixel and for the pixel belonging tothe dark tone, by giving priority to the arrangement of the blank dot,the paper white portion conspicuous in the shadow area is made highlydispersive. Next, an aspect is explained as a second embodiment in whichthe arrangement of the three kinds of dot, that is, the medium and smalldots plus the zero dot, is made highly dispersive in total in the shadowarea.

As described previously, in the shadow area, by making the arrangementof the blank dot visually conspicuous highly dispersive, a visually goodimage is obtained. However, in the shadow area, not only the blank dotcorresponding to the paper white portion, but also the actual dot thatis light compared to the image density (dot for which the ink ejectionamount is relatively small) is likely to be conspicuous visually.Further, the dot for which the ink ejection amount is small has a smalldot diameter on the paper surface and has a strong possibility of beingexposed as the paper white portion due to the ink landing positionshift. Consequently, a method is explained in which the arrangement ofthe medium and small dots plus the zero dot is determined in total sothat not only the blank dot but also the actual dot whose dot diameteris comparatively small becomes highly dispersive. Explanation of thebasic configuration and the separation processing unit of the imageforming apparatus in common to those of the first embodiment is omittedand in the following, the contents of the quantization processing unit,which is the different point, are explained mainly.

The internal configuration of the quantization processing unit 115 ofthe present embodiment is basically the same as that of the firstembodiment. That is, the quantization processing unit 115 includes thepriority order determination unit 204, the dot arrangement determinationunit 205, and the data output unit 206. However, the priority orderdetermination unit 204 of the present embodiment differs from that ofthe first embodiment in determining the priority order for each imageplane of CMYK in place of determining the priority order for each pixelof interest. Further, the dot arrangement determination unit 205 of thepresent embodiment differs from that of the first embodiment incomprising n error line buffers corresponding to each dot kind for whichink is ejected.

FIG. 9 is a flowchart showing the flow of the quantization processingaccording to the present embodiment. The quantization processing in thepresent embodiment is also the same for each color of CMYK, andtherefore, in the following, explanation is given by taking black (K) asa representative example. In the following explanation, symbol “S”represents a step.

First, at S901, the priority order determination unit 204 determines thepriority order (P1, P2, P3, P4) for the dot generation amount data onthe blank dot and each of the large, medium, and small dots generated inthe separation processing unit 114 for the entire K plane. In thepresent embodiment, it is assumed that importance is attached to theimprovement of the granularity in the shadow area and the priority orderis determined as P1=K_zero, P2=K_small, P3=K_medium, and P4=K_large.

Next, at S902 to 904, by using the total value of the dot generationamount data on the dots whose priority order is the top three, thequantization processing is performed for the pixel of interest and thedot arrangement of P1, P2, and P3 in total is determined.

At S902, a total value (hereinafter, described as “P123”) of the dotgeneration amount data P1 (x, y), P2 (x, y), and P3 (x, y) correspondingto the pixel of interest position (x, y) is calculated. The total valueP123 is expressed by formula (19) below.

P123=P1(x,y)+P2(x,y)+P3(x,y)  formula(19)

At S903, for the total value P123 of the dot generation amount datacorresponding to the pixel of interest position (x, y), itscorresponding error E_P123 (x) is read from the error line buffer andadded thereto. Total value data P123′ after the error is added isexpressed by formula (20) below.

P123′=P123+E_P123(x)  formula (20)

At S904, quantization by a comparison between P123′ and the thresholdvalue Th is performed. In a case where P123′ is greater than thethreshold value Th, the quantization results are ON (=255) and in a casewhere P123′ is less than or equal to the threshold value Th, thequantization results are OFF (=0). Here, it is assumed that thequantization results for P123′ are represented as O_P123. In thismanner, the dot arrangement that makes the three kinds of dot (here,each of zero, small, and medium dots) corresponding to P123 highlydispersive in total is determined. Then, for the rest of the highlydispersive arrangement of P1, P2, and P3, the dot arrangement of the dot(here, large dot) corresponding to P4 is determined.

Then at S905, whether the quantization results (O_P123) of P123′ are ONor OFF is determined. In a case where O_P123 is ON (=255), theprocessing advances to S906. On the other hand, in a case where O_P123is OFF (=0), the processing advances to S916.

Next, at S906 to S908, by using the total value of the dot generationamount data whose priority order is the top two, the quantizationprocessing for the pixel of interest is performed and the dotarrangement of P1 and P2 in total is determined.

At S906, a total value (hereinafter, described as “P12”) of the dotgeneration amount data P1 (x, y) and P2 (x, y) corresponding to thepixel of interest position (x, y) is calculated. The total value P12 isexpressed by formula (21) below.

P12=P1(x,y)+P2(x,y)  formula(21)

At S907, for the total value P12 of the dot generation amount datacorresponding to the pixel of interest position (x, y), itscorresponding error E_P12 (x) is read from the error line buffer andadded thereto. Total value data P12′ after the error is added isexpressed by formula (22) below.

P12′=P12+E_P12(x)  formula (22)

At S908, quantization by a comparison between P12′ and the thresholdvalue Th is performed. In a case where P12′ is greater than thethreshold value Th, the quantization results are ON (=255) and in a casewhere P12′ is less than or equal to the threshold value Th, thequantization results are OFF (=0). Here, it is assumed that thequantization results for P12′ are represented as O_P12. In this manner,the dot arrangement that makes the two kinds of dot (here, zero dot andsmall dot) corresponding to P12 highly dispersive in total isdetermined. Then, for the rest of the highly dispersive arrangement ofP1 and P2, the dot arrangement of the dot (here, medium dot)corresponding to P3 is determined.

Then, at S909, whether the quantization results (O_P12) of P12′ are ONor OFF is determined. In a case where O_P12 is ON (=255), the processingadvances to S910. On the other hand, in a case where O_P12 is OFF (=0),the processing advances to S915. Next, at S910 and S911, by using thedot generation amount data P1 whose priority order is the highest, thequantization processing for the pixel of interest is performed and thedot arrangement of P1 is determined.

At S910, for the dot generation amount data P1 (x, y) corresponding tothe pixel of interest position (x, y), its corresponding error E_P1 (x)is read from the error line buffer and added thereto. The data P1′ afterthe error is added is expressed by formula (23) below.

P1′=P1(x,y)+E_P1(x)  formula(23)

At S911, quantization by a comparison between P1′ and the thresholdvalue Th is performed. In a case where P1′ is greater than the thresholdvalue Th, the quantization results are ON (=255) and in a case where P1′is less than or equal to the threshold value Th, the quantizationresults are OFF (=0). Here, it is assumed that the quantization resultsfor P1′ are represented as O_P1.

Then, at S912, whether the quantization results (O_P1) of P1′ are ON orOFF is determined. In a case where O_P1 is ON (=255), the processingadvances to S913. On the other hand, in a case where O_P1 is OFF (=0),the processing advances to S914.

At S913 to S916 that follow, processing to set the quantization resultsof P1, P2, P3, and P4 at the pixel of interest position (x, y) to ON(here, as the quantized value, 255 is set) is performed. That is, atS913, O_P1 (x, y) is set to ON, at S914, O_P2 (x, y) is set to ON, atS915, O_P3 (x, y) is set to ON, and at S916, O_P4 (x, y) is set to ON.

Next, at S917 and S918, quantization errors produced by theabove-described quantization processing are diffused in accordance withpredetermined error diffusion coefficients and stored in the error linebuffer.

At S917, differences between the pixel values P123′, P12′, and P1′before the quantization processing is performed and each of thequantization results O_P123, O_P12, and O_P1 are calculated. Here, in acase where the differences to be calculated are taken to be ERR123,ERR12, and ERR1, they are expressed by formula (24) to formula (26)below, respectively.

ERR123=P123′−O_P123(x,y)  formula (24)

ERR12=P12′−O_P12(x,y)  formula (25)

ERR1=P1′−O_P1(x,y)  formula (26)

At S918, the calculated errors are diffused to the pixels around thepixel of interest in accordance with the predetermined error diffusioncoefficients (see FIG. 7 described previously). FIG. 10A to FIG. 10Ceach show an example of the error line buffer of the present embodiment.FIG. 10A is the error line buffer corresponding to the total value P123of the dot generation amount data P1 (x, y), P2 (x, y), and P3 (x, y)corresponding to the pixel of interest position (x, y). Then, FIG. 10Bis the error line buffer corresponding to the total value P12 of P1 (x,y) and P2 (x, y) and FIG. 10C is the error line buffer corresponding toP1 (x, y). Each error line buffer has (W (number of horizontal pixels inthe input image)+1) storage areas (E_P123 (x), E_P12 (x), E_P1 (x),here, x=1 to (W+1)). Then, the quantization error is stored in eachstorage area. Specifically, the errors EEP123, ERR12, and ERR1 in P123,P12, and P1 are stored in each corresponding error line buffer asfollows.

E_P123(x+1)=E_P123(x+1)+ERR123× 7/16(x<W)

E_P123(x−1)=E_P123(x−1)+ERR123× 3/16(x>1)

E_P123(x)=E_P123(W+1)+ERR123× 5/16(1<x<W)

E_P123(x)=E_P123(W+1)+ERR123× 8/16(x=1)

E_P123(x)=E_P123(W+1)+ERR123× 13/16(x=W)

E_P123(W+1)=ERR123× 1/16(x<W)

E_P123(W+1)=0(x=W)

E_P12(x+1)=E_P12(x+1)+ERR12× 7/16(x<W)

E_P12(x−1)=E_P12(x−1)+ERR12× 3/16(x>1)

E_P12(x)=E_P12(W+1)+ERR12× 5/16(1<x<W)

E_P12(x)=E_P12(W+1)+ERR12× 8/16(x=1)

E_P12(x)=E_P12(W+1)+ERR12× 13/16(x=W)

E_P12(W+1)=ERR12× 1/16(x<W)

E_P12(W+1)=0(x=W)

E_P1(x+1)=E_P1(x+1)+ERR1× 7/16(x<W)

E_P1(x−1)=E_P1(x−1)+ERR1× 3/16(x>1)

E_P1(x)=E_P1(W+1)+ERR1× 5/16(1<x<W)

E_P1(x)=E_P1(W+1)+ERR1× 8/16(x=1)

E_P1(x)=E_P1(W+1)+ERR1× 13/16(x=W)

E_P1(W+1)=ERR1× 1/16(x<W)

E_P1(W+1)=0(x=W)

At S919 that follows, the data output unit 206 outputs the dotarrangement data corresponding to the actual large, medium, and smalldots that the line head can form based on the priority order informationreceived from the priority order determination unit 204. Here, accordingto the priority order information, the priority order is P1=K_zero,P2=K_small, P3=K_medium, and P4=K_large. In this case, as the inkejection dot arrangement data on the pixel of interest position (x, y),the following three pieces of data are output to the image forming unit120.

-   -   Large dot: O_P4 (x, y)    -   Medium dot: O_P3 (x, y)    -   Small dot: O_P2 (x, y)

Then, at S920, whether or not the processing for all the pixels of the Kplane is completed is determined. In a case where there is anunprocessed pixel, the processing returns to S902, and the next pixel istaken as the pixel of interest and the processing is continued. On theother hand, in a case where the processing of all the pixels iscompleted, this processing is terminated.

The above is the contents of the quantization processing by the methodof the present embodiment. FIG. 11A and FIG. 11B are diagrams explainingthe effects of the quantization processing by the method of the presentembodiment. FIG. 11A is an example in which the dot arrangement isdetermined so that the three kinds of dot, that is, the medium and smalldots plus the zero dot, become highly dispersive in total in the darktone and FIG. 11B is an example in which the dot arrangement isdetermined so that the three kinds of dot, that is, the large, medium,and small dots, become highly dispersive in total for comparison. Fromthe comparison between both examples, it is known that the dot patternin FIG. 11B is low in dispersity in the dot whose size is small (whosedensity level is low).

In a case where the dot separation table as shown in FIG. 4 describedpreviously is used by attaching importance to improvement of granularityin the shadow area as in the present embodiment, the large dot is notused and ink dots are formed only by the medium dot and the small dot ineach pixel in the highlight area. However, in a case where it is desiredto attach importance to, for example, suppression of streaking(banding), a dot separation table that mixes also the large dot for thepixel in the highlight area is used sometimes. In this case, it ispossible to obtain a good image by arranging the three kinds of actualdot, that is, the large, medium, and small dots, so that they are highlydispersive in total. Because of this, it is better to change thedetermination criterion of the priority order as appropriate inaccordance with to which of improvement of granularity and suppressionof streaking importance is attached by the image quality desired by auser. For example, in a case where it is desired to attach importance tosuppression of streaking, it is sufficient to determine the priorityorder to be set for each processing-target image plane, for example, asP1=K_large, P2=K_medium, P3=K_small, and P4=K_zero.

According to the present embodiment, the arrangement of the pixel thatis light and conspicuous in the dark tone becomes highly dispersive, andtherefore, it is possible to obtain an image of good granularity.

Third Embodiment

In the first and second embodiments, the dot generation amount data onthe blank dot corresponding to the paper white portion is generatedbased on the dot generation amount data on the n kinds of actual dot.Next, an aspect is explained as a third embodiment in which the dotgeneration amount data on the n kinds of actual dot and the dotgeneration amount data on the blank dot area generated at the same timeby using a dot size separation table including information on the blankdot. Explanation of the contents in common to those of the first andsecond embodiments is omitted and in the following, different points areexplained mainly.

FIG. 12 is a functional block diagram showing the internal configurationof a separation processing unit 114′ and the quantization processingunit 115. As shown in FIG. 12, the separation processing unit 114′includes the color separation processing unit 201 and a dot generationamount determination unit 1201. The quantization processing unit 115includes the priority order determination unit 204, the dot arrangementdetermination unit 205, and the data output unit 206 as in the firstembodiment. In the following, along the flowchart shown in FIG. 13,processing to generate the dot generation amount data on the actual dotsand the dot generation amount data on the blank dot at the same timeaccording to the present embodiment is explained. In the followingexplanation, symbol “S” represents a step.

At S1301, the multi-valued RGB image data that the image formingapparatus 100 has received from an external information processingapparatus (not shown schematically) is input to the color separationprocessing unit 201 of the separation processing unit 114′. At S1302that follows, the color separation processing unit 201 performs colorseparation processing for the RGB image data and coverts it into CMYKimage data by using a color separation table prepared in advance. Theobtained CMYK image data is sent to the dot generation amountdetermination unit 1201.

At S1303, the dot generation amount determination unit 1201 performs dotsize separation processing for the CMYK image data by using a dotseparation table prepared in advance. In the present embodiment, eachimage plane of CMYK is separated into four kinds of dot (large, medium,and small dots plus zero dot) plane. FIG. 14 is a diagram showing anexample of the dot separation table used in the present embodiment.Different from the dot separation table in FIG. 4 described previously,also for the blank dot, the dot generation amount for each tone isdescribed. By using the dot separation table such as this, which alsoincludes the information indicating the generation amount for each toneof the blank dot, each image plane of CMYK is separated into the fourkinds of dot (large, medium, and small dots plus zero dot) plane at thesame time. Then, by using the four kinds of dot generation amount data,the quantization processing explained in the first and secondembodiments is performed at next S1304 and the dot arrangement in eachimage plane of CMYK is determined.

Modification Example

In the present embodiment, “color separation processing” and “dotgeneration amount data determination processing of actual dots plusblank dot” are performed separately, but it may also be possible toperform the processing at the same time. In a case where the processingis performed at the same time, for example, a separation table is storedin advance, in which values for each of (n+1) kinds of dot for each ofCMYK corresponding to each RGB value are stored. Then, by referring tothe separation table, the input RGB image data is separated into fourkinds of dot plane for each of CMYK as shown below.

-   -   C_large, C_medium, C_small, C_zero    -   M_large, M_medium, M_small, M_zero    -   Y_large, Y_medium, Y_small, Y_zero    -   K_large, K_medium, K_small, K_zero

Further, it may also be possible to design the configuration so that thedot generation amount data for each dot size to which the priority orderis attached is generated directly from the CMYK image data after colorseparation by providing a processing unit that integrates the dotgeneration amount determination unit 1201 and the priority orderdetermination unit 204. A case where this way of thinking is applied tothe second embodiment will be, for example, as follows. First, a dotseparation table in accordance with the image quality desired by a user(importance is attached to granularity or to suppression of streaking)is prepared. Further, criterion information on the priority order isprepared separately, which gives the highest priority to the blank dotin a case where importance is attached to granularity or to the largedot in a case where importance is attached to suppression of streaking.Then, in accordance with which a user attaches importance to, the dotseparation table and the priority order criterion information to be usedare switched and four pieces (P1 to P4) of dot generation amount dataare output. For example, in a case where importance is attached togranularity, in accordance with the dot separation table describing thedot generation amount of each dot and the priority order criterioninformation that gives the highest priority to the blank dot, each pieceof dot generation amount data is output, that is, P1 for the zero dot,P2 for the small dot, P3 for the medium dot, and P4 for the large dot.In a case where importance is attached to suppression of streaking,determination of the dot generation amounts of the actual large, medium,and small dots naturally determines the dot generation amount of theremaining blank dot, and therefore, in this case, output may beterminated after the three pieces of dot generation amount data on P1 toP3 are output.

As described above, in the present embodiment, by using the separationtable for each dot size, which also includes information on the blankdot, as shown in FIG. 14, the dot generation amount data on each of(n+1) kinds of dot is generated at the same time. In a case of thepresent embodiment, the blank dot generation amount determination unitis no longer necessary, and therefore, it is possible to furthersimplify the configuration of the separation processing unit. Further,it is possible to specify the generation amount of the blank dot indetail for each tone, and therefore, it is made possible to control thepixel of the paper white portion more directly.

Fourth Embodiment

In the first to third embodiments, by using the dot generation amountdata corresponding to each of the large, medium, and small dots plus thezero dot, the dot arrangement is determined in order from the dot whosepriority order is the highest. That is, the dot arrangement isdetermined on the premise that each of the (n+1) kinds of dot iscompletely exclusive of one another. Next, an aspect is explained as afourth embodiment in which the complete exclusion is not premised at thetime of determining the arrangement of each of the (n+1) kinds of dotand the overlap between dots of different kinds is permitted. In thefollowing, explanation is given based on the first embodiment anddifferent points are explained mainly.

FIG. 15 is a flowchart showing the flow of quantization processing thatpermits the overlap between dots of different kinds in the dotarrangement determination unit 205 according to the present embodiment.This flowchart is different from the flowchart in FIG. 6 of the firstembodiment in that the steps corresponding to S604, S607, and 610 do notexist. In the following, explanation is given along the flow in FIG. 15.In the following explanation, symbol “S” represents a step.

First, as at S601, for the pixel of interest of the processing-targetimage plane, the priority order (P1, P2, P3, P4) of the four kinds(large, medium, and small dots plus zero dot) of dot generation amountdata generated in the separation processing unit 114 is determined(S1501). Next, to the dot generation amount data P1 (x, y) correspondingto the pixel of interest position (x, y), its corresponding error E_P1(x) is added and the dot generation amount data P1′ after the error isadded is acquired (S1502). Then, quantization by a comparison betweenP1′ and the threshold value Th is performed (S1503). After this,similarly, the processing is performed for P2 (x, y), P3 (x, y), and P4(x, y). That is, to the dot generation amount data P2 (x, y)corresponding to the pixel of interest position (x, y), itscorresponding error E_P2 (x) is added and the dot generation amount dataP2′ after the error is added is acquired (S1504). Then, quantization bya comparison between P2′ and the threshold value Th is performed(S1505). Next, to the dot generation amount data P3 (x, y) correspondingto the pixel of interest position (x, y), its corresponding error E_P3(x) is added and the dot generation amount data P3′ after the error isadded is acquired (S1506). Then, quantization by a comparison betweenP3′ and the threshold value Th is performed (S1507). Further, to the dotgeneration amount data P4 (x, y) corresponding to the pixel of interestposition (x, y), its corresponding error E_P4 (x) is added and the dotgeneration amount data P4′ after the error is added is acquired (S1508).Then, quantization by a comparison between P4′ and the threshold valueTh is performed (S1509).

Following the above, errors that are differences between the values P1′,P2′, P3′, and P4′ before the quantization processing is performed andeach of the quantization results are calculated (S1510) and thecalculated errors are diffused to the pixels around the pixel ofinterest in accordance with predetermined error diffusion coefficients(S1511).

Then, based on the priority order, the dot arrangement datacorresponding to each of the large, medium, and small dots is output(S1512). As described previously, in a case of the line head, it is onlypossible to eject an amount of ink corresponding to one kind of dot forthe same position from one nozzle. Because of this, the data output unit206 determines one target dot based on the priority order informationamong each of the large, medium, and small dots plus the zero dot andoutputs the arrangement data on the dot. That is, the data output unit206 determines one kind of dot in accordance with the priority orderinformation and outputs the arrangement data on the dot. For example,for the pixel belonging to the dark tone, P1 is for the zero dot, andtherefore, except for this, the dot arrangement data on the actual dotsis output in the priority order of P2 for the small dot, P3 for themedium dot, and P4 for the large dot. For example, in a case where thedot arrangement data corresponding to P2 is output, the dot arrangementof P2 is output from the position that is the rest of the dotarrangement of P1 (among the dots of P2, only the dots at the positionthat does not overlap the dots of P1 are output). Further, similarly, ina case where the dot arrangement data corresponding to P3 is output, thedot arrangement of P3 is output from the position that is the rest ofthe dot arrangement of P1 and P2 (among the dots of P3, only the dots atthe position that does not overlap the dots of P1 and P2 are output).Furthermore, in a case where the dot arrangement data corresponding toP4 is output, the dot arrangement of P4 is output from the position thatis the rest of the dot arrangement of P1 to P3 (among the dots of P4,only the dots at the position that does not overlap the dots of P1 to P3are output). At this time, in the portion in which the overlap occurswith another dot arrangement, all the intended dots are not formed, andtherefore, the target density is not achieved in the final outputresults. Consequently, in a case of the present embodiment, it isdesirable to design the dot separation table by taking intoconsideration the possibility such as this and determine the dotgeneration amount of each dot.

The above-described processing is repeated for all the pixels (S1513).The above is the contents of the quantization processing by the methodof the present embodiment.

According to the present embodiment, also in a case where the dotarrangement is determined without premising that the dots of differentkinds are completely exclusive of one another, it is possible to obtainthe effects in the embodiments described previously.

Fifth Embodiment

In the first to fourth embodiments, the aspect is explained in which thearrangement of the blank dot corresponding to the paper white portion isdetermined with priority at the time of determining the arrangement ofthe plurality of different kinds of dot whose ink ejection amount (=dotsize) is different. Next, an aspect is explained as a fifth embodimentin which the arrangement of the blank dot corresponding to the paperwhite portion is determined with priority over the actual dots in eachcolor of CMYK.

In the following, explanation is given by taking a case as an examplewhere the method of the first embodiment is applied to the determinationof the dot arrangement for each color, but the configuration may be onein which the dot arrangement for each color is determined by applyingthe method of another embodiment.

In a case where the first embodiment is applied to the arrangementdetermination of the actual dots in each color of CMYK, theconfiguration of the separation processing unit 114 and the quantizationprocessing unit 115 may be the configuration shown in FIG. 2 describedpreviously. That is, the separation processing unit 114 includes thecolor separation processing unit 201, the actual dot generation amountdetermination unit 202, and the blank dot generation amountdetermination unit 203. Further, the quantization processing unit 115includes the priority order determination unit 204, the dot arrangementdetermination unit 205, and the data output unit 206. In the following,explanation is given along the flow in FIG. 3 explained in the firstembodiment.

In a case where printing-target color (RGB) image data is input (S301),the color separation processing unit 201 performs color separationprocessing for the RGB image data and converts it into CMYK image planescorresponding to each ink color (S302). Then, the actual dot generationamount determination unit 202 performs dot size separation processingfor the CMYK image data and generates dot generation amount data foreach size of the large, medium, and small dots (S303). Due to this, thefollowing twelve dot planes are obtained.

-   -   C plane→C_large, C_medium, C_small    -   M plane→M_large, M_medium, M_small    -   Y plane→Y_large, Y_medium, Y_small    -   K plane→K_large, K_medium, K_small

By using the total of twelve dot planes thus generated, the blank dotgeneration amount determination unit 203 generates dot generation amountdata D_zero corresponding to the blank dot corresponding to the paperwhite portion in which no actual dot in any color is formed (S304). Thatis, in the preceding embodiments, the zero dot generation amount data(C_zero, M_zero, Y_zero, K_zero) is generated for each color of CMYK,but in the present embodiment, the zero dot generation amount dataD_zero common to the colors is generated. In this case, accumulated dotgeneration amount data C_all, M_all, Y_all, and K_all on the large,medium, and small dots in each color of CMYK is expressed by formula(27) to formula (30) below, respectively.

C_all=C_large+C_medium+C_small  formula (27)

M_all=M_large+M_medium+M_small  formula (28)

Y_all=Y_large+Y_medium+Y_small  formula (29)

K_all=K_large+K_medium+K_small  formula (30)

Then, the zero dot generation amount data D_zero common to the colors isexpressed by formula (31) below.

D_zero=255−K_all−C_all−M_all−Y_all  formula (31)

In a case where D_zero is smaller than 0, D_zero is clipped to 0. Inthis manner, to the quantization processing unit 115, the accumulateddot generation amount data on the large, medium, and small dots in eachcolor of CMYK and the dot generation amount data on the blank dot commonto the colors are input.

Then, the quantization processing unit 115 performs the quantizationprocessing (S305) to give priority to the arrangement of the blank dotcorresponding to the paper white portion in the final output resultsobtained by superimposing each plane of CMYK after all the actual dotsare formed. FIG. 16 is a flowchart showing the flow of quantizationprocessing to determine the arrangement of each dot in each color bytaking the accumulated dot arrangement for each color as a restrictionin the dot arrangement determination unit 205 of the present embodiment.In the following, explanation is given along the flow in FIG. 16. In thefollowing explanation, symbol “S” represents a step.

First, at S1601, the priority order determination unit 204 determinesthe priority order for the accumulated dot generation amount data oneach color of CMYK and the zero dot generation amount data common to thecolors in the pixel of interest. Here, it is assumed that the priorityorder is represented as Px (x=1 to 5) in order from the highestpriority. Further, it is assumed that the dot generation amount data onthe large dot belonging to Px is represented as Px_large, the dotgeneration amount data on the medium dot as Px_medium, and the dotgeneration amount data on the small dot as Px_small. For example, in acase of the pixel of interest belong to the dark tone, Px is determinedas P1=D_zero, P2=K_all, P3=M_all, P4=C_all, and P5=Y_all. Due to this,it is possible to make the dot arrangement of the dot conspicuous in theshadow area highly dispersive with priority.

Then, at S1602 to S1618, the dot arrangement determination unit 205converts the accumulated dot generation amount data (Px) into binary dotarrangement data (O_Px) in accordance with the determined priorityorder. Specifically, as follows.

First, for the dot generation amount data P1 (x, y) corresponding to thepixel of interest position (x, y), its corresponding error E_P1 (x) isread from the error line buffer and added thereto (S1601). Then,quantization by a comparison between P1′ and the threshold value Th isperformed (S1603). In this manner, in the dark tone, the arrangement ofthe blank dot corresponding to P1 is determined with priority over theactual dots in each color.

Next, for the dot generation amount data (here, K_all) on P2 whosepriority order is the second highest, the quantization processing isperformed and the dot arrangement is determined. First, the correctionamount H based on the arrangement determination results of the dot(here, zero dot) whose priority order is higher is calculated and theobtained correction amount H is reflected in the dot generation amountdata P2 (x, y) corresponding to the pixel of interest position (x, y)(S1604). Then, for the dot generation amount data P2 (x, y)corresponding to the pixel of interest position (x, y), itscorresponding error E_P2 (x) is read from the error line buffer andadded thereto (S1605). Then, quantization by a comparison between P2″and the threshold value Th is performed (S1606).

Next, for the dot generation amount data (here, M_all) on P3 whosepriority order is the third highest, the quantization processing isperformed and the dot arrangement is determined. First, the correctionamount H based on the arrangement determination results of the dots(here, zero and K dots) whose priority order is higher is calculated andthe obtained correction amount H is reflected in the dot generationamount data P2 (x, y) corresponding to the pixel of interest position(x, y) (S1607). Then, for the dot generation amount data P3 (x, y)corresponding to the pixel of interest position (x, y), itscorresponding error E_P3 (x) is read from the error line buffer andadded thereto (S1608). Then, quantization by a comparison between P3″and the threshold value Th is performed (S1609).

Next, for the dot generation amount data (here, C_all) on P4 whosepriority order is the fourth highest, the quantization processing isperformed and the dot arrangement is determined. First, the correctionamount H based on the arrangement determination results of the dots(here, zero, K, and M dots) whose priority order is higher is calculatedand the obtained correction amount H is reflected in the dot generationamount data P3 (x, y) corresponding to the pixel of interest position(x, y) (S1610). Then, for the dot generation amount data P4 (x, y)corresponding to the pixel of interest position (x, y), itscorresponding error E_P4 (x) is read from the error line buffer andadded thereto (S1611). Then, quantization by a comparison between P4″and the threshold value Th is performed (S1612).

Next, for the dot generation amount data (here, Y_all) on P5 whosepriority order is the fifth highest, the quantization processing isperformed and the dot arrangement is determined. First, the correctionamount H based on the arrangement determination results of the dots(here, zero, K, M, and C dots) whose priority order is higher iscalculated and the obtained correction amount H is reflected in the dotgeneration amount data P4 (x, y) corresponding to the pixel of interestposition (x, y) (S1613). Then, for the dot generation amount data P5 (x,y) corresponding to the pixel of interest position (x, y), itscorresponding error E_P5 (x) is read from the error line buffer andadded thereto (S1614). Then, quantization by a comparison between P5″and the threshold value Th is performed (S1615).

Following the above, differences between the values P1″, P2″, P3″, P4″,and P5″ before the quantization processing is performed and each of thequantization results O_P1, O_P2, OP3, OP4, and O_P5 are calculated(S1616). Then, the calculated errors are diffused to the pixels aroundthe pixel of interest in accordance with the error diffusioncoefficients described previously (S1617). Specifically, the errorsERR_P1, ERR_P2, ERR_P3, ERR_P4, and ERR_P5 in P1 to P5 are storedrespectively in the corresponding error buffers.

Then, whether or not the processing for all the pixels of the inputimage is completed is determined (S1618). In a case where there is anunprocessed pixel, the next pixel is taken as the pixel of interest andthe processing is continued. On the other hand, in a case where theprocessing of all the pixels is completed, the processing advances toS1619.

Next, based on the dot arrangement data (O_Px (x=1 to 5)) and the dotgeneration amount data on each of the large, medium, and small dots(Px_large, Px_medium, Px_small (x=1 to 5)), the dot arrangement data onthe large, medium, and small dots for each of CMYK is generated (S1619).For the determination of the arrangement of the large, medium, and smalldots for each color by taking the already-determined dot arrangementdata (O_Px (x=1 to 5)) as a restriction, it may be possible toappropriately use the conventional technique disclosed in, for example,Patent Literature 2. In a case where the target dot arrangement dataO_Px is the dot arrangement data on the blank dot, which does not havethe dot generation amount data on the large, medium, and small dots,this is regarded as a case to which this processing is not applied, andtherefore, this processing is skipped.

Then, based on the priority order information, the dot arrangement datacorresponding to each of the large, medium, and small dots that the linehead can form is output (S1620). In a case where the priority orderinformation is P1=D_zero, P2=K_all, P3=M_all, P4=C_all, and P5=Y_all,the dot arrangement data on the large, medium, and small dots is outputin order from black.

The above is the contents of the quantization processing according tothe method of the present embodiment. According to the presentembodiment, it is possible to obtain an image whose dispersity of thedot arrangement between colors is high and whose granularity is good.

Modification Example

In the first to fifth embodiments, the quantization processing isperformed based on the priority order information indicating thepriority order. However, it is not necessarily required to generate thepriority order information based on the dot generation amount data inorder to implement the same processing. For example, for the image planecorresponding to each color of CMYK, the tone value of each pixel iscompared with a predetermined threshold value. In a case where the tonevalue is less than the predetermined threshold value, the quantizationprocessing unit that quantizes the dots in order of the large dot, themedium dot, the small dot, and the blank dot quantizes the generationamount of each dot in the pixel of interest. In a case where the tonevalue is greater than or equal to the predetermined threshold value, thequantization processing unit that quantizes the dots in order of theblank dot, the small dot, the medium dot, and the large dot quantizesthe generation amount of each dot in the pixel of interest. By changingthe processing into the branched processing such as this, it is possibleto implement the same processing by a configuration that does notrequire the processing of the priority order information.

Further, in the first to fifth embodiment, for all the pixels, theprocessing to calculate the dot generation amount data on the blank dotcorresponding to the paper white portion is performed. However, in acase of the dark tone, it is desirable to arrange the black dot withpriority so that the dispersity becomes high. Consequently, it may alsobe possible to determine the arrangement of each dot by using only thedot generation amount data corresponding to the conventional actual dotsin a case where the tone is in the light portion, and apply theabove-described embodiments in a case where the tone is in the darkportion. Alternatively, for the pixel whose tone is in the lightportion, also by setting the data indicating the generation amount ofthe blank dot to “0”, it is possible to simplify the processing in thelight portion.

Further, in the first to fifth embodiments, in a case of the dark tone,the blank dot corresponding to the paper white portion is arranged withpriority over the other actual dots. However, for example, in a casewhere the printing medium is not white and the printing medium is in acolor such as gray, the dot that is likely to be conspicuous in the darktone may be in order of the small dot, the blank dot, the medium dot,and the large dot. In such a case, it is sufficient to determine thepriority order in the dark tone in order of the small dot, the blankdot, the medium dot, and the large dot.

Sixth Embodiment

Next, an aspect is explained as a sixth embodiment in which thearrangement of the blank dot is determined with priority by thequantization processing and the actual dots for which ink is ejectedactually are arranged at the position that is the rest of thearrangement position of the blank dot. In the present embodiment,explanation is given by taking a case as an example where imageformation is performed with a single dot size in the image plane in eachink color. FIG. 17 is a functional block diagram showing the internalconfiguration of the separation processing unit 114 and the quantizationprocessing unit 115 according to the present embodiment. As shown inFIG. 17, the separation processing unit 114 comprises an actual dotgeneration amount acquisition unit 1701 and a blank dot generationamount determination unit 1702. The actual dot generation amountacquisition unit 1701 acquires a tone image having the same kinds ofcolor, the same number of colors, and the same resolution as those ofthe ink colors ejected in the image forming unit 120. The blank dotgeneration amount determination unit 1702 determines the generationamount of the blank dot corresponding to the paper white portion inwhich no actual dot is formed. The quantization processing unit 115comprises a dot arrangement determination unit 1703 and a data outputunit 1704. The dot arrangement determination unit 1703 performs thequantization processing by the error diffusion method for the blank dotgeneration amount data and determines the arrangement of the blank dot.The data output unit 1704 outputs the dot arrangement data on the actualdots that the line head can form based on the blank dot arrangement datareceived from the dot arrangement determination unit 1703. In thefollowing, the processing of each unit shown in FIG. 17 is explained indetail along the flowchart shown in FIG. 18. In the followingexplanation, symbol “S” represents a step.

At S1801, the actual dot generation amount acquisition unit 1701acquires the dot generation amount data on the actual dots from apreprocessing unit, not shown schematically. That is, it is assumed thatin the previous stage of the actual dot generation amount acquisitionunit 1701, by the preprocessing unit, not shown schematically, thenecessary preprocessing, such as color conversion and resolutionconversion, is performed and image data after the preprocessing isacquired. In this case, the dot generation amount data on the actualdots is the image data having the same kinds of color, the same numberof colors, and the same resolution as those of the ink colors used inthe image forming unit 120. For example, in a case of the ink jet methodthat implements an output resolution of 1,200 dpi by using four colorinks of cyan (C), magenta (M), yellow (Y), and black (K), multi-valuedimage data having eight bits (256 tones) for each color of CMYK isacquired as the dot generation amount data on the actual dots. Thesubsequent processing is the same for each color of CMYK, and therefore,in the following, explanation is given by taking black (K) as arepresentative example.

At S1802, the blank dot generation amount determination unit 1702determines the generation amount of the blank dot corresponding to thepaper white portion in which no actual dot is formed based on the dotgeneration amount data on the actual dots acquired at S1801.Specifically, the dot generation amount data on the blank dot isobtained by calculating “255−actual dot generation amount” for eachpixel based on the acquired dot generation amount data on the actualdots. The dot generation amount data on the blank dot thus obtained issent to the quantization processing unit 115.

At S1803, the dot arrangement determination unit 1703 performs thequantization processing by the error diffusion method for the dotgeneration amount data on the blank dot and determines the dotarrangement of the blank dot. Due to this, the dot arrangement data onthe blank dot in which ON or OFF of the dot is indicated for each pixelis obtained. Details of the quantization processing here will bedescribed later. The dot arrangement data on the blank dot obtained bythe quantization processing is sent to the data output unit 1704.

At S1804, the data output unit 1704 outputs the dot arrangement data onthe actual dots that can be formed by the line head based on the dotarrangement data on the blank dot received from the dot arrangementdetermination unit 1703. Specifically, the dot arrangement data on theactual dots is obtained by reversing (changing ON to OFF, OFF to ON) thedot arrangement data on the blank dot for each pixel.

The above is the outline of the processing in the separation processingunit 114 and the quantization processing unit 115 according to thepresent embodiment.

(Details of Quantization Processing)

Following the above, details of the quantization processing at S1803 areexplained along the flowchart in FIG. 19. Here, explanation is given bytaking a case as an example where the quantization processing for thedot generation amount data on the blank dot is performed by the errordiffusion method. In this case, in order to diffuse and accumulateerrors, the error line buffers corresponding to the blank dot areprepared. All the error line buffers are initialized by an initial value(=0) or a random value before the start of the processing.

At S1901, for the dot generation amount data P1 (x, y) corresponding tothe pixel of interest position (x, y), its corresponding error E_P1 (x)is read from the error line buffer and added thereto. The dot generationamount data P1′ after the error is added is expressed by formula (32)below.

P1′=P1(x,y)+E_P1(x)  formula(32)

Then, at S1902, quantization by a comparison between P1′ and thethreshold value Th is performed. In a case where P1′ is greater than thethreshold value Th, the quantization results are ON and in a case whereP1′ is less than or equal to the threshold value Th, the quantizationresults are OFF. Here, it is assumed that the quantization results forP1′ are represented as O_P1. In this manner, the arrangement of theblank dot corresponding to P1 is determined with priority over theactual dots.

Next at S1903, a difference between the value P1′ before thequantization processing is performed and the quantization results O_P1is calculated as a quantization error. At S1904 that follows, thecalculated quantization error (P1′−O_P1) is diffused (stored in theerror line buffer) to the pixels around the pixel of interest inaccordance with the error diffusion coefficients.

Then, at S1905, whether or not the processing for all the pixels of theK plane is completed is determined. In a case where there is anunprocessed pixel, the processing returns to S1901, and the next pixelis taken as the pixel of interest and the processing is continued. Onthe other hand, in a case where the processing of all the pixels iscompleted, this processing is terminated.

The above is the contents of the quantization processing by the methodof the present embodiment.

Modification Example

In the present embodiment, explanation is given by taking the case ofthe quantization processing using the error diffusion method as anexample, but the quantization processing using the dither method may beaccepted. In a case by the dither method, a dither matrix obtained byarranging different threshold values within a matrix of a predeterminedsize is prepared in advance, and this dither matrix is sequentiallyloaded in tiles on the input image data and the tone value of the inputimage data and the corresponding threshold value are compared inmagnitude. Then, in a case where the tone value is greater than thethreshold value, the dot is turned ON and in a case where the tone valueis less than threshold value, the dot is turned OFF. Compared to theerror diffusion method, the dither method has an advantage in that theprocessing speed is high and it is possible to design the configurationby a simple circuit because the feedback of the quantization error isnot necessary.

The quantization processing in a case by the dither method is explainedwith reference to FIG. 20A to FIG. 20C. In this case, it is assumed thatthe dot generation amount data on the blank dot shown in FIG. 20A isrepresented as P1 and the tone value at the pixel position (x, y) in P1is represented as P1 (x, y).

For the pixel of interest position (x, y) in the dot generation amountdata P1 in FIG. 20A, the tone value P1 (x, y) of the dot generationamount data on the blank dot and the corresponding threshold value in adither matrix M (x, y) shown in FIG. 20B are compared in magnitude.Then, in a case where the tone value is greater than the thresholdvalue, a value (ON) indicating that the blank dot is arranged isdetermined as the output value and in other cases, a value (OFF)indicating that the bland dot is not arranged is determined as theoutput value. Here, in the dot generation amount data on the blank dotshown in FIG. 20A, the tone value at any pixel position is “33”.Consequently, at the pixel position at which the threshold value in thedither matrix M shown in FIG. 20B is smaller than or equal to “32”, theblank dot is determined to be ON. The dot arrangement determination unit1703 applies the processing such as this repeatedly to the whole of theinput image data while moving the position in the dither matrix anddetermines the arrangement of the blank dot in the determined generationamount. FIG. 20C shows an output image as the final quantization resultsand the black pixel represents ON of the blank dot and the white pixelrepresents OFF of the blank dot.

As above, it is also possible to implement the quantization processingin the dot arrangement determination unit 1703 by the dither method.

Modification Example

In the present embodiment, explanation is given by taking the case as anexample where image formation is performed with a single dot size in theimage plane in each ink color, but the present embodiment is not limitedto this. It is possible to deal with a plurality of dot sizes byperforming distribution again so that the dot arrangement in the dotarrangement data on the actual dots of the single dot size, which isoutput by the data output unit 1704, changes into the dot arrangement ofactual dots of a plurality of dot sizes. As the distribution method, forexample, the pixel whose dot is ON in the dot arrangement data on theactual dots of the single dot size is distributed randomly in the imageplane for each size of the large, medium, and small dots. Alternatively,it may also be possible to perform distribution by using the errordiffusion method or the dither method so that the dispersity becomesgood. Further, it may also be possible to perform distribution by takinga predetermined distribution ratio determined in advance as thedistribution ratio to a plurality of dot sizes, or it may also bepossible to change the distribution ratio in accordance with the dotgeneration amount of the blank dot. As regards the change in density dueto the distribution to a plurality of dot sizes, it is possible toadjust the output density for the input value by performing densitycorrection in the processing unit or the like that performs ycorrection, which is included in the image processing unit 110.

In the following, an example of a case is explained where distributionto a plurality of dot sizes is performed in a predetermined ratio basedon the dot generation amount of the blank dot by using the dithermethod. Here, in the output image shown in FIG. 20C, from the positionof the white pixel at which the blank dot becomes OFF, the dotarrangement of the actual dots of a plurality of dot sizes isdetermined. Specifically, at the pixel position at which the thresholdvalue of the dither matrix is greater than or equal to the pixel valueof the dot generation amount data P1 on the blank dot, the actual dotsof the single dot size are allocated to the actual dots of each dotsize. At this time, for example, the dot at the pixel position at whichthe corresponding threshold value within the dither matrix is greaterthan or equal to P1 and less than or equal to (128+P1/2) is determinedto be the small dot. Then, the dot at the position at which neitherblank dot nor small dot is placed (the pixel position at which thecorresponding threshold value within the dither matrix is greater than(128+P1/2)) is determined to be the large dot, and so on. By the methodsuch as this, it is possible to apply the present embodiment also to theactual dots of a plurality of dot sizes.

As above, in the present embodiment, the arrangement of the blank dot isdetermined with priority by the quantization processing and the actualdots for which ink is ejected actually are arranged at the position thatis the rest of the position of the arrangement of the blank dot. Due tothis, the arrangement of the blank dot is determined with priority overthe actual dots, and therefore, it is possible to determine thearrangement of the actual dots in which the dot dispersity of the blankdot is good. In particular, in the dark portion, the paper white islikely to be visually recognized as the blank dot, and therefore, it ispossible to keep the higher dispersity in the dark tone. In the presentembodiment, the determination of the arrangement is performed for thedata on each color of CMYK, but it may also be possible to determine theactual dot arrangement based on the arrangement of the blank dot in apart of the colors, for example, such as only in K with which the blankdot is likely to be conspicuous in the dark tone.

Seventh Embodiment

A printing medium, such as paper, has a limit (hereinafter, called “inkduty limit”) to a total amount of ink that can be applied per unit area.In a case where ink is ejected onto a printing medium by an amountexceeding the ink duty limit, the ink is not absorbed by the printingmedium and flooding is caused and resulting in the deterioration of theimage quality. Because of this, in order to secure the printing quality,it is necessary to suppress the ink application amount to within therange of the ink duty limit. Consequently, an aspect is explained as aseventh embodiment based on the six embodiment, in which the arrangementof the blank dots is determined so as to be highly dispersive by takinginto consideration the ink duty limit in accordance with a printingmedium.

FIG. 21 is a functional block diagram showing the internal configurationof the separation processing unit 114 and the quantization processingunit 115 according to the present embodiment. As shown in FIG. 21, theseparation processing unit 114 of the present embodiment comprises anink duty limit setting unit 2101, in addition to the actual dotgeneration amount acquisition unit 1701 and the blank dot generationamount determination unit 1702 shown in FIG. 17 described previously.There is no difference in the internal configuration of the quantizationprocessing unit 115.

The ink duty limit setting unit 2101 sets an ink duty limit inaccordance with the kind of printing medium (plain paper, glossy paper,mat paper, and the like) in order to secure printing quality. Thesetting method is not limited and for example, it may also be possiblefor a user to set an ink duty limit by specifying an arbitrary value viathe operation panel (not shown schematically) in the image formingapparatus 100 or a printer driver installed in an external PC. Further,it may also be possible for a corresponding ink duty limit to be setautomatically by storing in advance a table in which a printing mediumtype to be used and an ink duty limit are associate with each other andby a user specifying a printing medium type.

In the following, along the flowchart shown in FIG. 22, the processingin each unit shown in FIG. 21 is explained in detail. However,explanation is given by omitting or simplifying explanation of theportions in common to those of the flow in FIG. 18 of the sixthembodiment. In the following explanation, symbol “S” represents a step.

At S2201, the actual dot generation amount acquisition unit 1701acquires dot generation amount data on the actual dots having the samekinds of color, the same number of colors, and the same resolution asthose of the ink colors used in the image forming unit 120 from apreprocessing unit, not shown schematically. The subsequent processingis the same for each color of CMYK, and therefore, in the following,explanation is given by taking black (K) as a representative example.

At S2202, the ink duty limit setting unit 2101 sets an ink duty limitcorresponding to a printing medium to be used based on userinstructions. As described above, the ink duty limit indicates the totalamount of ink that can be applied per unit area, which is specified foreach printing medium, and it can be said that the ink duty limitrepresents the maximum value of the dot generation amount of the actualdots.

At S2203, the blank dot generation amount determination unit 1702determines the dot generation amount data on the blank dot based on thedot generation amount data on the actual dots acquired at S2201 and theink duty limit set at S2202. Specifically, a value obtained bycalculating “255−actual dot generation amount” based on the dotgeneration amount data on the actual dots and a value obtained bycalculating “255−ink duty limit value” based on the maximum value of thedot generation amount of the actual dots, which is indicated by the inkduty limit, are compared for each pixel. Then, the larger value isdetermined as the dot generation amount of the blank dot. The dotgeneration amount data on the blank dot thus obtained is sent to thequantization processing unit 115.

At S2203, the dot arrangement determination unit 1703 determines the dotarrangement of the blank dot by performing the same quantizationprocessing as that at S1803 for the blank dot generation amount data.Due to this, the dot arrangement data on the blank dot is obtained, inwhich ON or OFF of the dot is indicated for each pixel. The dotarrangement data on the blank dot obtained by the quantizationprocessing is sent to the data output unit 1704.

At S2204, the data output unit 1704 outputs the dot arrangement data onthe actual dots that can be formed by the line head based on the dotarrangement data on the blank dot received from the dot arrangementdetermination unit 1703. Specifically, the dot arrangement data on theactual dots is obtained by reversing (changing ON to OFF, OFF to ON) thedot arrangement data on the blank dot for each pixel.

The above is the outline of the processing in the separation processingunit 114 and the quantization processing unit 115 according to thepresent embodiment. In the method described above, the quantizationprocessing for the blank dot is performed separately, and therefore, thequantization processing is performed one more time corresponding to theone plane. However, by defining the plane whose generation amount is thelargest among the dot generation amounts of each dot including the blankdot as a plane whose priority order is the lowest and performing the dotarrangement to the plane last, it is possible to omit the quantizationprocessing for the plane. Consequently, it is also possible to implementthe present method, like the conventional method, by n pieces ofquantization processing for n plane inputs.

As above, in the present embodiment, the dot generation amount data onthe blank dot is determined based on the dot generation amount data onthe actual dots and the ink duty limit in accordance with a printingmedium. Further, the arrangement of the blank dot is determined withpriority by the quantization processing and the actual dots are arrangedat the position that is the rest of the position of the arrangement ofthe blank dot. Due to this, it is possible to determine the dotarrangement of the actual dots in which the dot dispersity of the blankdot is good while securing the printing quality.

OTHER EMBODIMENTS

Embodiment(s) of the present invention can also be realized by acomputer of a system or apparatus that reads out and executes computerexecutable instructions (e.g., one or more programs) recorded on astorage medium (which may also be referred to more fully as a‘non-transitory computer-readable storage medium’) to perform thefunctions of one or more of the above-described embodiment(s) and/orthat includes one or more circuits (e.g., application specificintegrated circuit (ASIC)) for performing the functions of one or moreof the above-described embodiment(s), and by a method performed by thecomputer of the system or apparatus by, for example, reading out andexecuting the computer executable instructions from the storage mediumto perform the functions of one or more of the above-describedembodiment(s) and/or controlling the one or more circuits to perform thefunctions of one or more of the above-described embodiment(s). Thecomputer may comprise one or more processors (e.g., central processingunit (CPU), micro processing unit (MPU)) and may include a network ofseparate computers or separate processors to read out and execute thecomputer executable instructions. The computer executable instructionsmay be provided to the computer, for example, from a network or thestorage medium. The storage medium may include, for example, one or moreof a hard disk, a random-access memory (RAM), a read only memory (ROM),a storage of distributed computing systems, an optical disk (such as acompact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™),a flash memory device, a memory card, and the like.

According to the image processing apparatus according to the presentinvention, it is possible to implement a better granularity.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

1. An image processing apparatus that generates image data to be outputto an image forming apparatus printing an image by an ink dot formed byejecting ink onto a printing medium, the image processing apparatuscomprising: a generation unit configured to generate, based on an inputimage, first generation amount data indicating a generation amount ofeach of one or more kinds of ink dot for each pixel and secondgeneration amount data indicating a generation amount of a blank dot forwhich the ink dot is not formed for each pixel; and a processing unitconfigured to determine a dot arrangement pattern indicating anarrangement of each of the one or more kinds of ink dot by performingquantization processing using the first generation amount data and thesecond generation amount data.
 2. The image processing apparatusaccording to claim 1, wherein the generation unit generates the firstgeneration amount data by determining a generation amount of each of theone or more kinds of ink dots in accordance with a tone value of eachpixel in the input image.
 3. The image processing apparatus according toclaim 2, wherein the image forming apparatus can eject n kinds (n≥2) ofink dot of different sizes for the ink, the generation unit generatesthe first generation amount data on each of the n kinds of ink dot, andthe processing unit determines the dot arrangement pattern based on thesecond generation amount data for at least one kind of ink dot of the nkinds of ink dot.
 4. The image processing apparatus according to claim1, wherein a maximum value that can be taken as a pixel value of thefirst generation amount data and the second generation amount data isthe same and the generation unit calculates a value obtained bysubtracting a pixel value of a pixel in the first generation amount datafrom the maximum value for each pixel as the generation amount of theblank dot.
 5. The image processing apparatus according to claim 3,wherein the processing unit quantizes, in a case where a tone value ofthe input image indicates a dark portion, a blank dot with priority overat least one ink dot of the n kinds of ink dot.
 6. The image processingapparatus according to claim 3, wherein the processing unit determines apriority order of the n kinds of ink dot and the blank dot and theprocessing unit determines an arrangement of each dot in accordance witha determined priority order.
 7. The image processing apparatus accordingto claim 6, wherein the processing unit determines the priority orderfor each pixel of the input image.
 8. The image processing apparatusaccording to claim 7, wherein the n kinds of ink dot is classified inaccordance with a density level of a tone value in each pixel in theinput image and the processing unit determines a priority order of theblank dot to be higher than that of the n kinds of ink dot for a pixelbelonging to a dark tone among each of the pixels.
 9. The imageprocessing apparatus according to claim 8, wherein the processing unitdetermines, in a case where the first generation amount of a dot whosedensity level is higher among the n kinds of ink dot is larger than orequal to the second generation amount of the blank dot, a priority orderof the blank dot to be higher than that of the n kinds of ink dot foreach of the pixels.
 10. The image processing apparatus according toclaim 8, wherein the processing unit determines, in a case where thefirst generation amount of a dot whose density level is higher among then kinds of ink dot is larger than 0, a priority order of the blank dotto be higher than that of the ink dot for each of the pixels.
 11. Theimage processing apparatus according to claim 6, wherein the processingunit determines an arrangement of each dot so that the blank dot becomeshighly dispersive for a pixel belonging to a dark tone among each pixelof the input image.
 12. The image processing apparatus according toclaim 6, wherein the processing unit determines an arrangement of eachdot so that the blank dot and a dot whose density level is lower amongthe n kinds of ink dot become highly dispersive in total for a pixelbelonging to a dark tone among each pixel of the input image.
 13. Theimage processing apparatus according to claim 6, wherein the n kinds ofink dot are classified in accordance with a density level of a tonevalue in each pixel of an input image and formed for each color of aplurality of color materials and the processing unit determines thepriority order for each image plane corresponding to the plurality ofcolor materials.
 14. The image processing apparatus according to claim13, wherein the processing unit: determines a priority order of theblank dot to be higher than that of a dot whose density level is higheramong the n kinds of ink dot in a case where importance is attached togranularity in an image represented by the n kinds of ink dot; anddetermines a priority order of a dot whose density level is higher amongthe n kinds of ink dot to be higher than that of the blank dot in a casewhere importance is attached to suppression of streaking in an imagerepresented by the n kinds of ink dot.
 15. The image processingapparatus according to claim 3, wherein the generation unit generatesthe first generation amount data on the n kinds of ink dot by using aseparation table describing the first generation amount for each tone ofeach of then kinds of ink dot.
 16. The image processing apparatusaccording to claim 1, wherein the generation unit generates generationamount data on the one or more kinds of ink dot and the blank dot byusing a separation table describing a generation amount for each tone ofeach of the one or more kinds of ink dot and the blank dot.
 17. Theimage processing apparatus according to claim 16, wherein the processingunit determines an arrangement of each dot so that the one or more kindsof ink dot and the blank dot are exclusive of one another.
 18. Aprinting medium on which an image is formed by at least one or morekinds of ink dot, wherein in a dark area, dispersity of a dotcorresponding to paper white for which the one or more kinds of ink dotare not ejected is higher than dispersity of at least one dot among theone or more kinds of ink dot and in a light area, dispersity of at leastone dot among the one or more kinds of ink dot is higher than dispersityof a dot corresponding to paper white.
 19. A method of generating imagedata to be output to an image forming apparatus printing an image by anink dot formed by ejecting ink onto a printing medium, the methodcomprising: a step of generating, based on an input image, firstgeneration amount data indicating a generation amount of each of one ormore kinds of ink dot for each pixel and second generation amount dataindicating a generation amount of a blank dot for which the ink dot isnot formed for each pixel; and a processing step of determining a dotarrangement pattern indicating an arrangement of each of the one or morekinds of ink dot by performing quantization processing using the firstgeneration amount data and the second generation amount data.
 20. Anon-transitory computer-readable storage medium storing a program forcausing a computer to perform a method of generating image data to beoutput to an image forming apparatus printing an image by an ink dotformed by ejecting ink onto a printing medium, the method comprising: astep of generating, based on an input image, first generation amountdata indicating a generation amount of each of one or more kinds of inkdot for each pixel and second generation amount data indicating ageneration amount of a blank dot for which the ink dot is not formed foreach pixel; and a processing step of determining a dot arrangementpattern indicating an arrangement of each of the one or more kinds ofink dot by performing quantization processing using the first generationamount data and the second generation amount data.